diff --git a/dumux/discretization/cellcentered/mpfa/omethod/localassembler.hh b/dumux/discretization/cellcentered/mpfa/omethod/localassembler.hh
index 50116c73c6426a33c0309c797891c1dc7c801144..7cb4fa1b2e099d7da53f608782919e51bd052782 100644
--- a/dumux/discretization/cellcentered/mpfa/omethod/localassembler.hh
+++ b/dumux/discretization/cellcentered/mpfa/omethod/localassembler.hh
@@ -591,7 +591,7 @@ public:
                 const auto& wijk = iv.omegas()[localScvfIdx][idxInOutside+1];
 
                 // make sure the given outside gravity container has the right size
-                assert(outsideG[pIdx][localScvfIdx].size() == iv.localScvf(localScvfIdx).neighboringLocalScvIndices().size());
+                assert(outsideG[pIdx][localScvfIdx].size() == iv.localScvf(localScvfIdx).neighboringLocalScvIndices().size()-1);
 
                 // add contributions from all local directions
                 for (LocalIndexType localDir = 0; localDir < IV::Traits::GridView::dimension; localDir++)